Transcoder, method of transcoding, and digital recorder

Abstract

A transcoder reduces the processing amount when a coded stream to which at least intra prediction is applied is transcoded into a coded stream to which intra prediction and inter prediction are applied. The transcoder converts a first coded stream to at least part of which intra prediction is applied, into a second coded stream to which intra prediction and inter prediction are applied. The transcoder includes a decoding unit that decodes the first coded stream to generate decoded image data; a prediction mode obtaining unit that obtains a prediction mode of intra prediction applied in decoding; and a coding unit that codes the decoded image data by applying the intra prediction or inter prediction to generate the second coded stream. A control unit controls causes the coding unit to use the prediction mode obtained by the prediction mode obtaining unit when the coding unit applies intra prediction.

The invention claimed is: 
     
       1. A transcoder which converts a first coded stream to at least part of which intra prediction is applied, into a second coded stream to which intra prediction and inter prediction are applied, the transcoder comprising:
 a decoding unit configured to decode the first coded stream on a per predetermined unit basis to generate decoded image data; 
 an intra prediction direction obtaining unit configured to obtain an intra prediction direction on a per processing unit basis, the intra prediction direction being one of intra prediction directions (i) specified in a coding standard used in the coding of the first coded stream and (ii) actually applied to the first coded stream, the processing unit being a processing unit of the intra prediction in coding of the first coded stream; 
 a coding unit configured to code the decoded image data by applying intra prediction or inter prediction on a per prediction processing unit basis to generate the second coded stream; and 
 a control unit configured to control the coding unit so that the coding unit reuses the intra prediction direction obtained by the intra prediction direction obtaining unit in the decoding of the first coded stream performed by the decoding unit, when the coding unit performs the prediction processing using the intra prediction; and 
 a first cost value generating unit configured to generate a first cost value on the per predetermined unit basis based on decoded information generated in a decoding process performed by the decoding unit and before intra prediction or inter prediction is applied, the first cost value indicating coding efficiency when intra prediction is to be applied, 
 wherein the control unit is further configured to control the coding unit so that the coding unit applies intra prediction or inter prediction according to the first cost value on the per predetermined unit basis when coding the decoded image data. 
 
     
     
       2. The transcoder according to  claim 1 ,
 wherein the first cost value generating unit is configured to generate the first cost value that is smaller as the coding efficiency when intra prediction is applied is higher, and 
 the control unit is configured to control the coding unit so that the coding unit applies intra prediction when the first cost value is smaller than a first threshold. 
 
     
     
       3. The transcoder according to  claim 2 ,
 wherein the coding unit includes: 
 an inter prediction unit configured to generate inter prediction image data by applying inter prediction to the decoded image data on the per predetermined unit basis; 
 a second cost value generating unit configured to generate a second cost value on the per predetermined unit basis based on the inter prediction image data, the second cost value indicating coding efficiency when inter prediction is applied; 
 an intra prediction unit configured to generate intra prediction image data by applying using the intra prediction direction, intra prediction on the per predetermined unit basis; and 
 a third cost value generating unit configured to generate a third cost value on the per predetermined unit basis based on the intra prediction image data, the third cost value indicating coding efficiency when intra prediction is applied, and 
 the control unit is configured to control, when the first cost value is equal to or larger than the first threshold, the coding unit so that the coding unit compares the second cost value and the third cost value and applies intra prediction or inter prediction based on a result of the comparing. 
 
     
     
       4. The transcoder according to  claim 3 ,
 wherein the control unit is configured to control the coding unit so that the coding unit generates the second cost value after determining that the first cost value is equal to or larger than the first threshold and compares the second cost value and the third cost value. 
 
     
     
       5. The transcoder according to  claim 3 ,
 wherein the control unit is configured to control the coding unit so that the coding unit compares the second cost value and the third cost value and applies intra prediction when the coding efficiency indicated by the third cost value is higher than the coding efficiency indicated by the second cost value. 
 
     
     
       6. The transcoder according to  claim 1 , further comprising
 a coding parameter generating unit configured to generate a coding parameter based on the first cost value, the coding parameter used for setting at least one of a search range of motion estimation and the number of reference images. 
 
     
     
       7. The transcoder according to  claim 6 ,
 wherein the coding parameter generating unit is configured to count a result of comparing the first cost value and the first threshold and to generate the coding parameter according to a result of the counting. 
 
     
     
       8. The transcoder according to  claim 1 ,
 wherein the coding unit includes: 
 an intra prediction unit configured to generate intra prediction image data by applying using the intra prediction direction, intra prediction on the per predetermined unit basis; 
 a third cost value generating unit configured to generate a third cost value on the per predetermined unit basis based on the intra prediction image data, the third cost value being smaller as coding efficiency when intra prediction is applied is higher; 
 an inter prediction unit configured to generate inter prediction image data by applying inter prediction to the decoded image data on the per predetermined unit basis; and 
 a second cost value generating unit configured to generate a second cost value on the per predetermined unit basis based on the inter prediction image data, the second cost value indicating coding efficiency when inter prediction is applied, and 
 the control unit is configured to control the coding unit so that the coding unit generates the inter prediction image data and the second cost value after determining that the third cost value is equal to or larger than a second threshold. 
 
     
     
       9. A method of transcoding for converting a first coded stream to at least part of which intra prediction is applied, into a second coded stream to which intra prediction and inter prediction are applied, the method comprising:
 decoding the first coded stream on a per predetermined unit basis to generate decoded image data; 
 obtaining an intra prediction direction on a per processing unit basis, the intra prediction direction being one of intra prediction directions (i) specified in a coding standard used in the coding of the first coded stream and (ii) actually applied to the first coded stream, the processing unit being a processing unit of the intra prediction in coding of the first coded stream; 
 coding the decoded image data by applying intra prediction or inter prediction on a per prediction processing unit basis to generate the second coded stream; and 
 controlling reuse of the intra prediction direction obtained by the intra prediction direction obtaining step in the decoding of the first coded stream performed by the decoding step, when the coding step performs the prediction processing using the intra prediction; 
 a first cost value generating step of generating a first cost value on the per predetermined unit basis based on decoded information generated in a decoding process performed and before intra prediction or inter prediction is applied, the first cost value indicating coding efficiency when intra prediction is to be applied, 
 wherein the controlling step applies intra prediction or inter prediction according to the first cost value on the per predetermined unit basis when coding the decoded image data.